The deep (visceral) arterial system and the portal vein will be monitored (a) by catheters incorporating Doppler-shifted ultrasound transducers passed percutaneously into the adjacent veins; and the descending thoracic aorta and, possibly, the right pulmonary artery from the esophagus. The feasibility of transcutaneous Doppler-shifted ultrasound monitoring of deep vessels will be explored. Basic evaluation of the signals obtained will be carried out by sonar spectral analysis and the significance of each part of the pulse wave complex further investigated. In vitro, this will be done by altering individual physical factors influencing flow in elastic tubes. In vivo, signal- variation will be obtained by altering the state of the circulation and the results compared to those obtained by conventional standard methods. In particular the renal circulation will be examined under normal and induced abnormal conditions. Quantitation of blood flow will be a major objective. Human application will be carried out when instrumentation is perfected, and animal observations indicate the method may be applied safely and with benefit. The results will be evaluated against other methods for quality, effectiveness, cost and patient disturbance.